Ford #15 Mustang Boss 302 Trans Am - Add a 6-speed Gearbox

I really enjoy the recently added 1970 Ford #15 Mustang Boss 302 Trans Am! I’ve built mine for use in Multiclass races, specifically in B-class. It has great handling and a good amount of power.
The only thing I struggle with is the standard (non upgradable) 4-speed racing transmission. With the limited amount of gears you always have to sacrifice launch/acceleration/top speed, which is annoying.

Please give this car the upgrade of a 6-speed racing transmission!

Picture above is the #16, have a cool picture of the #15 instead:

I think this topic can be applied to all older racing cars as a lot of them have a 4- or 5-speed transmission. Having an option to upgrade them to 6-speed even though they already feature a racing transmission would be nice.